For centuries people have sought ways to separate style from
substance, good manners from good science, and user interface from
application logic. The Model View Controller (MVC) design pattern is a
way of separating the user-interface from the substance of the
application.

In recent years, MVC has become a popular strategy for building
websites. There are now web-MVC frameworks available
for many programming languages, for instance Struts
for Java, Maypole for Perl and Rails for Ruby.

This article introduces the MVC pattern and its use in web
development. I will mention some Perl modules, but most
of it will be applicable to any language supporting object-oriented
programming.

MVC is an object-oriented design pattern, so I'll mention objects,
classes and other patterns here. This text will probably not make
very much sense without some experience in OO programming and web
development. Information about all the patterns mentioned here
can be found at the C2 Wiki: http://c2.com/cgi/wiki

Since MVC was originally invented for traditional GUI applications,
certain details in original MVC pattern don't map well to web
applications. (Note 1)
Since I'm describing MVC for the web, I will simplify, change,
misrepresent and ignore those details here.

The Model handles the state of the application. The state is what
your application is about.
If your application is a forum, your Model might contain Class::DBI
objects
representing threads, users and postings. The Model does not know
anything about
HTML, or web servers or anything like that. It just supplies ways to
query the state, and ways to change that state.

The View is the representation of the user interface. Usually there are
many (possibly nested) Views in a single application. A view can query the
model, but it is not supposed to change the state. In web based MVC
systems, a view can be implemented using a template that renders
an HTML page. In our hypothetical forum application, the Views would be the
templates for rendering a full thread, the login page, the posting page
etc.

Since the Controller is in charge of handling the requests and
selecting an appropriate page (View), there is no immediate coupling
between the request made by the user and the resulting page.

This turns out to be very useful if the page-flow in the application is
complex, but even for simple applications the Controller is a good
place to handle common actions - authentication and session management
can be handled in the Controller, for instance.

Since all code that does anything except building a nice page for the
user is outside the View objects, changing the layout does not involve
touching the logic of the application. Since the part of the application
that changes the most during and after development is the layout,
this means much less chance of adding bugs.

Since all actions on the application state are handled by the Model,
it is possible to change the Model's implementation without touching
the user interface, as long as the Model's public API doesn't change.
(but see Coupling between View and Model).

Sometimes is just hard to figure out where a specific piece of the
application is supposed to go. Especially dividing the
Model from the Controller can be hard. As a rule of thumb, the Controller
should be as minimal as possible - it is only responsible for translating
HTTP requests into Model actions and selecting the right View - the
Model should provide all the behavior
it can without handling the HTTP requests or output formatting details.

One problem with having the View and Controller querying the Model
is that changing the Model's public API means you also have to adapt
the Controller and Views that act on it (Note 2). Adapting the Controller is
usually not too much work, but changing a large number of Views
will be annoying.

The Model Model View Controller pattern tries to minimize the impact
of these changes by using two Models: a Domain Model and an Application
Model - the View only queries the Application Model, and the Application
model can query the Domain Model. The Application Model usually
partly generated by the GUI design tools. I haven't used this pattern at
all, so I don't know how useful it is for web applications

Creating an MVC application can result in more classes and objects than
a ``page-based'' system. That means more design up front. On the
other hand, a well designed MVC system will be easier to adapt and expand,
because the code will be separated better.

Possibly :-) I think that any web-based application that uses more than
a handful of templates, or has a complex interaction between pages would
be a good candidate for the MVC pattern. There are alternatives, of course.
See Resources - Alternatives.

CGI::Application, by Jesse Erlbaum is a relatively simple module
that provides a good base-class for a Controller.
It's easy to understand and it has some convenient HTML::Template support
(though it doesn't force you to use HTML::Template if you like another
templating system better).

You derive your Controller from the CGI::Application
class, handling user requests via the run_modes system
and you can use a templating system to implement the Views.

How you implement your Model is entirely up to you. IMO this is a Good
Thing; the model is the most specific thing in an application, and a
framework should not make unnecessary restrictions on it.

Maypole, a fairly new project by Simon Cozens is a complete MVC
framework that uses lots
of other CPAN modules: CGI::Untaint, Class::DBI and extensions,
Template::Toolkit etc.
It ``aims to be the most generic and extensible ( ... ) MVC-based web
application framework'', and comes bundled with templates
for building CRUD (Create Update Delete) applications. Maypole runs best
on mod_perl (mod_perl 2 now supported).

While CGI::Application's approach is to set up a minimal base for
your Controller, giving the programmer as much
choice as possible, Maypole's focus is on rapid development: the
example BeerDB application requires about 20 lines of Perl and
a database schema. If you're building a CRUD application on mod_perl,
Maypole might be just what you're looking for.

All this convenience comes at a price, of course - Maypole is big,
slow to load (can be a problem in CGI environment), and if you
don't want it to auto-generate a whole CRUD application based on your
database schema, it takes a lot more time to figure out than
CGI::Application.

Update

borisz mentioned that Apache AxKit is also an MVC framework. I apologize for missing it. I've never used it, so I can't really tell much about it, though. From the website:

Apache AxKit is an XML Application Server for Apache. It provides on-the-fly conversion from XML to any format, such as HTML, WAP or text using either W3C standard techniques, or flexible custom code. AxKit also uses a built-in Perl interpreter to provide some amazingly powerful techniques for XML transformation.

Update - but wait, there's more!

Yes, I left out more MVC - based systems. perrin points to a list containing server web application frameworks.
Of that list, at least Apache::PageKit, and OpenInteract follow the MVC pattern. Check it out:

In ``traditional'' MVC applications, Views and Controllers register
their interest in parts of the Model and will then be notified when
those parts change. (Via the Observer pattern).
This assumes a permanent, single model and persistent views,
which is fine if your application
is a single process on a single machine, but becomes
cumbersome when you spread an application over
multiple processes or machines. Most web based MVC frameworks
use a passive Model instead, where the Controller and View request
information from the Model when they need it.

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